A. Field of the Invention
The invention relates generally to a perpendicular magnetic recording medium for use in a fixed magnetic recording device and a method for manufacturing the same, and more particularly to a perpendicular magnetic recording medium which can achieve an excellent read/write performance at a low medium noise, and a method for manufacturing the same.
B. Description of the Related Art
In recent years, personal computers, workstations, and similar devices have been equipped with a mass-storage and a small-sized magnetic recording apparatus, and a magnetic disk as a magnetic recording medium has been required to have increased recording density.
At present, an in-plane (longitudinal) magnetic recording method in which an easy axis is parallel to the surface of a magnetic recording medium is usually used, and as the surface recording density increases, the thermal stability of recording bits becomes a problem. To further increase the recording density, the use of perpendicular magnetic recording is being investigated. In a perpendicular magnetic recording medium, an easy axis of a magnetic film is aligned perpendicularly to the surface of a substrate. For this reason, magnetizations that are adjacent to each other do not face each other in a magnetization transition area. Even if the bit length is decreased, magnetization is stable, and magnetic fluxes are not decreased as is the case with an in-plane magnetic recording medium. Therefore, the perpendicular magnetic recording medium is suitable as a high density magnetic recording medium.
As stated above, a perpendicular magnetic recording medium is more advantageous as a high recording density magnetic recording medium than an in-plane magnetic recording medium, but causes high media noise. Ordinarily, as the media noise decreases, read/write performance is improved to achieve high-density recording, and hence a layer structure for a perpendicular magnetic recording medium has been devised to reduce media noise.
As an example of the above known perpendicular magnetic recording medium, as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-1203306,a “double-layer perpendicular magnetic recording medium” is known in which a soft magnetic backing layer is formed on a non-magnetic substrate such as aluminum or glass, an underlayer for perpendicularly aligning a magnetic layer is formed on the soft magnetic backing layer, and a perpendicular magnetic recording layer and a protective layer are formed on the underlayer. As this perpendicular magnetic recording layer, many multilayer film structures have been envisaged, such as a perpendicular magnetic film formed of a Co group alloy such as Co—Cr, Co—Cr—Ta, or Co—Cr—Pt, a multi-layered perpendicular magnetic film such as CO/Pt or Co/Pd in which a predetermined number of Co layers and Pt layers (or Pd layers) are alternately layered, and an amorphous perpendicular magnetic film such as Tb—Co or Tb—Fe—Co.
In particular, as a prospective high recording density medium, a great deal of research has been done with multi-layered perpendicular magnetic films such as Co/Pt or Co/Pd, since these have high perpendicular magnetic anisotropy (Ku), thermal stability, and coercivity, and can easily achieve a squareness ratio of approximately 1.0. However, a multi-layered perpendicular magnetic film such as Co/Pt causes high media noise, and various attempts have been made to reduce this media noise. For example, in Japanese Laid-Open Patent Publication (Kokai) 2002-25032,a method has been proposed in which B (boron) chips are placed on the surface of a target, and Argon (Ar) and oxygen (O2) are sputtered in a mixed gas to segregate boron (B) and an oxygen atom (O) at the grain boundary of a magnetic layer. According to this method, if the amount of boron (B) to be added is between 0 atom % and about 1.0 atom %, the signal-to-noise ratio (SNR) is improved, but if the amount of boron (B) added is greater, the SNR is saturated. Also, in the case where an oxygen atom (O) is added, if the amount of oxygen (O) to be added is between 0 atom % and about 0.1 atom %, the signal-to-noise ratio (SNR) is improved, but if the amount of oxygen atoms (O) to be added is greater, the SNR is saturated.
Also, in Japanese Laid-Open Patent Publication (Kokai) No. 2001-155329, a method has been proposed in which an underlayer or a segmented layer in which an oxide such as SiO2 is added to metal having a face-centered cubic structure such as Pt, Au, Pd, Ag, Rh, Ir, or Cu. According to this method, in Co layers and Pt layers (or Pd layers) constituting a magnetic layer, grains such as Co and Pt (or Pd) are not very far apart from each other, and hence there is a high magnetic interaction between crystal grains forming the magnetic layer, which causes media noise.
As described above, in a perpendicular magnetic recording medium having the conventional multi-layered film structure, media noise cannot be sufficiently reduced. In order to increase the surface recording density, media noise needs to be further reduced.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.